1. Field of the Invention
The present invention relates to a method for producing suspension parts of aluminum alloy by roll forming as preforming and ensuing forging in multiple stages.
2. Description of the Related Arts
Weight reduction of motor vehicles is desirable for better fuel efficiency. It is achieved by making bodies and parts from aluminum alloy. Light parts of suspension arms (referred to as suspension parts hereinafter) made of aluminum alloy reduce the unsprung mass, thereby improving roadability.
In general, suspension parts are complex in shape (asymmetrical about the axis) and they require high mechanical and fatigue strength (comparable to those of steel) as well as high impact resistance. Therefore, they are made of aluminum alloy (such as A6061 and 6000 series) by forging. The process of forming suspension parts from aluminum alloy is explained in the following.
First, aluminum alloy is formed into a round bar by extrusion or casting and the round bar is cut in lengths. The thus obtained workpiece undergoes preforming so that it has a volume distribution resembling the finished product. Then, the preformed workpiece undergoes forging in three stages. In the first stage, which is called buster, the preformed workpiece is heated to a prescribed temperature and then roughly forged by using a first die. Buster is followed by trimming and reheating. In the second stage, which is called blocker, the roughly forged workpiece is forged by using a second die which is closer to the finished shape than a first die. Blocker is followed by trimming and reheating. In the third stage, which is called finisher, the workpiece undergoes finish forging with a finish die. Finisher is followed by trimming.
Preforming is carried out by using such an apparatus as disclosed in Japanese Patent Laid-open No. 571/1994. This apparatus is so designed as to insert a rod-like workpiece in a die cavity and form it under pressure with a punch and a mandrel. During preforming, the workpiece has its wall thickness adequately controlled as the mandrel is moved to a desired height. (Adjustment of wall thickness may also be accomplished by using a mandrel consisting of several sections which are individually moved up and down.)
The workpiece for suspension parts should have dimensions which are established as illustrated in FIG. 9. FIG. 9(a) shows a suspension part 7R drawn by CAD (computer aided design). Suspension part 7R has an imaginary center line CR and a cross section FR (shown in FIG. 9(b)) which has the maximum sectional area. The cross section FR is taken along the line XRxe2x80x94XR in the direction of arrows in FIG. 9(a). An imaginary circle PR as shown in FIG. 9(c) is defined which has a diameter DR large enough for its circular area to cover the cross section FR plus flash. This imaginary circle PR is regarded as the cross-section of the workpiece 1R as shown in FIG. 9(c). Likewise, the imaginary center line CR passing through the suspension part 7R shown in FIG. 9(a) is regarded as the length LR of the workpiece 1R.
The disadvantage of the above-mentioned conventional process for producing suspension parts of aluminum alloy is that each of the three stages for pressing or forging gives rise to a large amount of flash which has to be removed at the end of each stage. Flash removal takes such a long time that it is necessary to reheat the workpiece before it undergoes the subsequent; step. The necessity for flash trimming and reheating aggravate productivity and increase production cost.
Moreover, the above-mentioned preforming apparatus is complex in structure and operation and hence is poor in productivity. In addition, it is constructed such that flash is liable to stick to the sliding part of the die and the split of the mandrel. This leads to low metal recovery and gives low-precision products. Flash removal requires additional maintenance cost.
The workpiece IR prepared as shown in FIG. 9 has a volume much larger than that of the suspension part 7R. Therefore, it prevents material shortage in preforming and forging but causes a large amount of flash which lowers metal recovery and productivity.
The present invention was completed to address the above-mentioned problem. It is an object of the present invention to provide a method for determining the dimensions of the workpiece for roll forming and a method for producing high-precision suspension parts efficiently from the workpiece. The workpiece is processed into the suspension part by preliminary roll forming and ensuring forging. Owing to its optimized shape, the workpiece gives only a minimum amount of flash without material shortage, thereby improving metal recovery and productivity.
The gist of the present invention resides in a method for producing suspension parts of aluminum alloy, said method comprising a step of preparing a workpiece for roll forming having prescribed dimensions, a step of heating said workpiece, a step of subjecting said workpiece, which has been heated, to roll forming, thereby giving a preformed workpiece, a step of subjecting said preformed workpiece to pressing (or forging) in multiple stages, thereby giving a formed product, and a step of subjecting the formed product to trimming, wherein said trimming is carried out only after the above-mentioned pressing in multiple stages has been completed.
According to the present invention, the preliminary workpiece is formed from a workpiece having prescribed dimensions by rolling for improved efficiency and operation. The pressing or forging in multiple stages is carried out such that trimming is carried out after all the stages are completed. Trimming in this way reduces operating time, and reduced operating time eliminates the necessity of reheating, which in turn reduces operating time further and production cost. In order that trimming is not required in the intermediate stages, the preformed workpiece should have an adequate shape which minimizes the amount of flash. This is easily accomplished if the preformed workpiece is prepared from a workpiece having prescribed dimensions by roll forming. Roll forming of a workpiece having prescribed dimensions to give the preformed workpiece and forging in multiple stages which needs trimming only at the last stage contribute to productivity when they are combined together.
The present invention produces its full effect when it is applied to suspension parts of aluminum alloy, because efficient and economical production is important for suspension parts to be produced in large quantities from aluminum alloy which is more expensive than iron. The present invention solves the problem with reduction of steps and improvement in metal recovery. The preforming by rolling is suitable for the workpiece for the suspension part which is comparatively thin and long and greatly varies in sectional area. Moreover, the preforming by rolling imparts grain flow in the lengthwise direction to the workpiece. This is desirable because aluminum greatly depends on grain flow for its mechanical properties (strength and toughness). After forging, the preformed workpiece with grain flow gives a suspension part which has also grain flow and hence has better mechanical properties than that obtained by pressing or forging alone. The method of the present invention may also be applied to any other parts of aluminum alloy (than suspension parts) which are comparatively thin and long and greatly vary in sectional area. Applying the method to such parts is also within the scope of the present invention.
Another gist of the present invention resides in a method for producing suspension parts of aluminum alloy as defined above, wherein the prescribed dimensions are determined by multiple steps of establishing an imaginary center line in the suspension part of aluminum alloy, acquiring plural orthogonal cross-sections perpendicular to the imaginary center line so as to be arranged along the imaginary center line, calculating a sectional area of the cross-sections, defining imaginary circles each having an imaginary area which is equal to the sectional area of the orthogonal cross-section plus a preliminarily established amount of flash, defining the imaginary area between said imaginary circles by interpolation, integrating said imaginary areas, thereby calculating the imaginary total volume of the suspension part of aluminum alloy, and obtaining the sectional area of the workpiece of roll forming based on the imaginary circle having the maximum area among said imaginary circles and dividing said total volume by said sectional area, thereby giving a value which is regarded as the length of the workpiece for roll forming.
The workpiece for roll forming which has dimensions determined as mentioned above permits efficient rolling. In addition, the workpiece for roll forming which has a volume plus an adequate amount of flash minimizes flash such that trimming is necessary only after the last stage of forging. Moreover, assuming the area of an imaginary circle having the maximum area is effective in preventing material shortage from occurring in the part where the sectional area of the suspension part is largest. According to the present invention, it is possible to determine accurately and rapidly the dimensions of the workpiece for roll forming.
When the dimensions of the workpiece for roll forming are determined, the step of defining the imaginary area between said imaginary circles by interpolation is carried out such that a circular truncated cone is formed between adjacent two imaginary circles.
When the dimensions of the workpiece for roll forming are determined, the sectional area of the workpiece for roll forming may be the maximum value of said imaginary area multiplied by a prescribed factor. The prescribed factor should preferably be 0.7 to 1.0. The factor in this range contributes to improvement in metal recovery. Incidentally, this factor has been established in consideration of the deforming characteristics of the workpiece, the entire shape of the suspension part, and the groove shape of the forming roll.
When the dimensions of the workpiece for roll forming are determined, the area of the orthogonal cross-section multiplied by a prescribed factor may be regarded as the amount of flash. In this case, the prescribed factor should preferably be 0.05 to 0.3. This factor is effective in preventing material shortage while controlling the amount of unnecessary flash.
The step of calculating the above-mentioned dimensions may be accomplished by using three-dimensional CAD data for the suspension part of aluminum alloy.
The gist of the present invention resides also in a method for producing suspension parts of aluminum alloy as defined in above, wherein the prescribed dimensions are determined by multiple steps of preparing a test piece having the volume of the workpiece, subjecting the test piece to preforming by actual roll forming and pressing (or forging) in multiple stages, decreasing the volume of the workpiece by a certain amount if trimming is necessary in the series of steps and subjecting the test piece again to preforming by actual roll forming and pressing (or forging) in multiple stages, increasing the volume of the workpiece by a certain amount if trimming is necessary in the series of steps and subjecting the test piece again to preforming by actual roll forming and pressing (or forging) in multiple stages, and repeating said steps until the prescribed conditions are satisfied.
In production of suspension parts of aluminum alloy as defined above, pressing (or forging) in multiple stages may be carried out simultaneously by using dies which are arranged side by side and mounted on the same pressing or forging machine.
According to the present invention, the workpiece for roll forming gives only at small amount of flash, so that there is no possibility of flash producing adverse effect when multiple stages of forging (such as buster, blocker, and finisher) are carried out simultaneously by using only a pressing or forging machine. Forging with dies arranged side by side contributes to efficient operation.